Field emitting device and method of making same

ABSTRACT

A non-thermionic field emitting device includes at least one pyramidal shaped field emitting element on one surface of an electrically conductive substrate. At least one needle-like projection is located on the tip of the pyramidal shaped field emitting element. An electron extracting electrode is mounted in parallel spaced relation to and is electrically insulated from the surface of the substrate having the field emitting element thereon. The electron extracting electrode has at least one aperture therein, the aperture being positioned substantially coaxially with a corresponding pyramidal shaped field emitting element.

United States Patent [191 Levine Nov. 18, 1975 FIELD EMITTING DEVICE AND METHOD OF MAKING SAME [75] Inventor: Jules David Levine, East Brunswick,

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: Sept. 3, 1974 [21] Appl. No.: 502,669

[52] US. Cl. 313/309; 313/336; 313/351 [51] Int. Cl. I-I0lJ 1/02 [58] Field of Search 313/309, 336, 351, 95

[56] References Cited UNITED STATES PATENTS 2,287,749 6/1942 Slayter 313/309 X 3,174,043 3/1965 Dyke et a1. 313/309 X 3,453,478 7/1969 Shoulders et al. 313/336 X 3,466,485 9/1969 Arthur, Jr. et al 313/309 X 3,665,241 5/1972 Spindt et al 313/309 Spindt et al 313/309 Nathanson et al 313/309 Primary ExaminerSaxfield Chatmon, Jr. Attorney, Agent, or Firm-Glenn H. Bruestle; William H. Murray [57] ABSTRACT A non-thermionic field emitting device includes at least one pyramidal shaped field emitting element on one surface of an electrically conductive substrate. At least one needle-like projection is located on the tip of the pyramidal shaped field emitting element. An electron extracting electrode is mounted in parallel spaced relation to and is electrically insulated from the surface of the substrate having the field emitting element thereon. The electron extracting electrode has at least one aperture therein, the aperture being positioned substantially coaxially with a correspondingpyramidal shaped field emitting element.

12 Claims, 18 Drawing Figures US. Patent Nov. 18, 1975- Sheet 1 of5 3,921,022

Patent Nov. 18, 1975 Sheet 4 of 5 INCREASING POTENTIAL ENERGY 0F ELECTRONS INCREASING POTENTIAL ENERGY 0F ELEGTRONS U.S.- Patent Nov. 18, 1975 Sheet5 0f5 3,921,022

INCREASING POTENTIAL ENERGY OF ELECTRONS FIELD EMITTING DEvIcE AND METHOD or MAKING SAME BACKGROUND OF THE INVENTION The present invention relates to field emitting devices and more particularly to non-thermionic field emitting devices and methods of making the same.

Non-thermionic field emitting devices are well known in the art. It is known that electron emission can be stimulated by an electric potential applied near a pointed cathode. The prior art sharply pointed field emitters can be broadly categorized by the type of material used for fabrication. One such category includes the use of semiconductor material such as silicon or germanium to construct arrays of sharply pointed field emitters. Although these devices appear to be particularly well suited for use as photodetectors, their use as large area cold cathode sources is severely limited. For example, the physical properties and cost of the single crystal semiconductor materials used limit the size of the arrays. In their paper entitled Fabrication and Some Applications of Large-Area Silicon Field Emission Arrays, Solid State Electronics, 1974, Vol. 17, pages 155-163, Thomas et al. consider large-area arrays to be on the order of cm In addition to being size limited, the current densities obtainable from semiconductor field emitters are less than those obtainable from metals.

Another prior art category of field emitting devices encompasses the use of sharply pointed metallic field emmitters. Typical of these devices is that disclosed in US. Pat. No. 3,755,704 issued to Spindt et al. These devices utilize individual needle-like protuberances deposited on an electrode, said protuberances being of a higher resistivity material than the electrodes. These devices suffer from two major disadvantages. First, the use of deposition techniques to form the protuberances limits the area over which uniform arrays can be formed. This technique includes projecting a source of emitter material onto a surface essentially normal to the surface while at the same time directing a source of masking material at the same surface at a shallow grazing angle from all sides. This is a critical operation which does not lend itself to use in forming very large quantities of emitter elements over very large surfaces. Second, the fabrication of these prior art devices also entails the use of thin film techniques which produce relatively delicate structures that are sensitive to strong electrical forces characteristics of field emission. In addition, the relative thinness of the insulators used in the prior art devices, typically on the order of 1 micron, can cause manufacturing problems in that a single pin hole in the insulation can ruin an entire field emitter array.

SUMMARY OF THE INVENTION A non-thermionic field emitting device comprises an electrically conductive substrate having opposed surfaces with at least one field emitting element on one of the opposed surfaces. The field emitting element has a pointed tip, with at least one needle-like projection thereon, which projects away from the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of one form of a field emitting device of the present invention.

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1.

FIGS. 3, 4, 5 and 6 are sectional views showing steps of making the field emitting device of the present invention.

FIG. 7 is a portion of an enlarged plan view of a sheet of electrically conductive material having a hole pat tern therein.

FIGS. 8, 9, 10. 11 and 12 are sectional views showing further steps of making the field emitting device of the present invention.

FIG. 13 is a cross section of one embodiment of the device of the present invention.

FIG. 14 is an electrical schematic diagram of the device depicted in FIG. 13.

FIGS. 15 and 16 are potential energy diagrams depicting the relative potential energies of electrons at different locations within the device depicted in FIG. 13.

FIG. 17 is another electrical schematic diagram of the device depicted in FIG. 13.

FIG. 18 is a potential energy diagram depicting the relative potential energies of electrons at different locations within the device shown in FIG. 13.

DETAILED DESCRIPTION Referring initially to FIGS. 1 and 2, one form of a nonthermionic field emitting device of the present invention is generally designated as 10. The field emitting device 10 comprises a substrate 12 of an electrically conductive material such as copper, having a matrix array of field emitting elements, generally referred to as 14, on one surface thereof. Each field emitting element 14 comprises a conically or a pyramidally shaped field emitter 16 with at least one needle-like projection hereinafter referred to as tiplets 18, located on the tip thereof. The field emitter l6 and the tiplets 18 are composed of a material having good field emission characteristics, such as copper. A layer of insulating material 20, such as glass, is on and bonded to the surface of the substrate having the field emitting elements 14 thereon. The insulating layer 20 has an array of apertures 21 therethrough which are positioned such that the insulating layer covers the surface of the substrate while leaving the field emitting elements 14 exposed through the apertures 21.

An electron extracting electrode 22, of an electrically conductive material such as a beryllium copper alloy is on and bonded to the insulating layer 20. The electron extracting electrode 22 has a plurality of apertures 24 therein, the number of apertures corresponding to the number of field emitting elements 14 in the array. The apertures are positioned such that each aperture is aligned substantially coaxially with a corresponding field emitting element.

To obtain the desired emission of electrons from the field emitting elements 14, the positive terminal of a voltage source 26 is connected to the electron extracting electrode 22 and the negative terminal is connected to the array of field emitting elements 14 through the substrate 12. Electrons are emitted from the tiplets 18 under the influence of the applied voltage. The emitted electrons pass through the apertures 24 in the electron extracting electrode 22 toward a suitable anode electrode such as a phosphor coated screen, not shown. The electron emitting device 10 of the present invention can consist of a single field emitting element 14 having an electron extracting electrode 22 with a single aperture therein in order to generate a single stream of electrons or it can consist of a large array of field emitting elements and electron extracting electrodes generating a large number of individually addressable electron streams.

To make the field emitting device 10, one starts with a substrate 12 of an electrically conductive material such as copper. One surface of the copper substrate is prepared in order to insure that the surface is substantially clean, flat and free from blemish prior to the application of a layer 28, see FIG. 3, of photosensitive, etch-resistant material or what is commonly known as a photoresist material. The photoresist material is then applied to the prepared surface and exposed to a light source through a transparency having an array of black dots. As shown in FIg. 3, the unexposed area of the photoresist layer 28 is washed away leaving an array of holes 30. It is to be noted here that, although the previous steps have been described using a negative photoresist material and transparency, these steps can be equally well carried out using a positive photoresist material and transparency and such embodiment is to be considered within the scope and contemplation of this invention. The surface of the copper substrate is then under etched through the holes 30, leaving an array of interconnected hemishperical valleys 32 as shown in FIG. 4.

Next, referring to FIG. 5, the photoresist layer 28 is stripped off leaving an array of mesa-like structures 34. The copper substrate is then oxidized, by any wellknown oxidizing method such as heating in air, forming a layer of copper oxide 36 having a thickness of approximately 2 mils. As shown in FIG. 6, the valleys 32 are then partially filled with a layer of an electrically insulating material 38 such as glass. One method of filling the valleys with glass is to place a sheet of glass approximately 3 mils thick across the tops of the mesalike structures 34 and place in a vacuum oven. A vacuum is drawn and the glass is heated until it becomes semi-molten and settles down into the valleys 32 leaving only a thin layer of glass 40 covering the tops of the mesa-like structures 34. The vacuum is used to substantially eliminate any air which may be trapped between the glass layer and the valleys 32.

Using standard photo etching techniques, a hole pattern 42, as partially shown in FIG. 7, is etched into a sheet of conductive material 44 such "as a beryllium copper alloy. The hole pattern 42 is laid out such that when the copper alloy sheet 44 is placed on the etched surface of the copper substrate 12, the holes 42 will be aligned with and surround the mesa-like structures 34. After the glass has settled into the valleys 32 it is slowly cooled to substantially room temperature. The copper alloy sheet 44 is then placed on the etched surface of the copper substrate 12 and positioned such that the mesa-like structures 34 protrude through the holes 42 while the remainder of the copper alloy sheet 44 is mounted on the glass insulating material 38 (see FIG. 8). The resulting structure is heated in order to bond the copper alloy to the glass insulating material 38. The structure is then removed from the oven and allowed to cool to substantially room temperature. Next, the thin layer of glass 40 which covers the mesa-like structures 34 is removed to expose the copper oxide 36. The copper oxide is then etched away to form pyramidally shaped field emitters 16 as shown in FIG. 9. These pyramidially shaped field emitters 16 typically have a tip diameter on the order of approximately 5 microns.

After the pyramidally shaped field emitters 16 are formed, a porous layer 46 of a material which does not form an appreciable oxide layer such as chromium. gold or rhodium, is deposited on the tips of said field emitters as shown in FIG. 10. The porous layer 46 in the preferred embodiment comprises chromium which has been electroplated on the tips of the field emitters using known porous plating techniques for example as described in the textbook Chromium by A. H. Sully, Butterworth, London (1954), Chapter 5. The chromium layer has pores or cracks 47 therethrough which are typically 1 micron apart. The field emitters 16 with the porous layer 46 deposited on the tips thereof, are then heated in air to oxidize those surfaces of the field emitters which are exposed beneath the pores or cracks 47. This oxidation process forms sharp points of copper covered by copper oxide beneath the solid portions of the porous layer 46 as indicated by the dotted line 48 in FIg. 11. After cooling to approximately room temperature, the oxide is chemically stripped from the tips of the field emitters 16 causing the porous layer 46 to fall off leaving an array of tiplets 18 exposed as shown in FIG. 12.

The operation of the device of the present invention is described making reference to FIG. 13 through 18. FIG. 13 shows a cross section of a single electron beam display device generally designated as 49 which includes a non-thermionic field emitting device 10 having as an electron souorce a single field emitting element 14 and an electron extracting electrode 22 having a single aperture 24 therein. Also included in the display device 49 is an electron target in the form of a phosphor coated display screen 50 and a screen electrode 52 having a mesh-like structure formed by a multiplicity of finely spaced apertures 54. The components of the display device 49 are joined together to form an airtight cavity 55 which is evacuated to provide a vacuum environment between the field emitting element 14 and the display screen 50. FIG. 14 is a schematic representation of the single emitter display device 49 depicted in FIG. 13 which has been connected to the external components required for basic device operation. The reference numeral used to identify the elements of the schematic diagram in FIG. 14 correspond to the reference numerals used to identify the parts of the embodiment depicted in FIg. 13.

As shown in FIG. 14, the positive terminal of a voltage source 26 is connected to the electron extracting electrode 22 and the negative terminal is connected to ground. The voltage applied to the electron extracting electrode 22 is on the order of 100 volts. The positive terminal of a first bias voltage source 56 is connected to the switch 58 and the negative terminal is connected to ground. The negative terminal of a second bias voltage aource 60 is connected to the switch 58 and the positive terminal is connected to ground. The voltage of the first 56 and second 60 bias voltage sources is typically 5 volts each. The positive terminal of a high voltage source 62 is connected to the anode electrode or phosphor coated screen 50 in this embodiment and the negative term nal is connected to ground. The voltage applied to the phosphor coated screen 50 is on the order of 20,000 volts.

FIGS. I5 and 16 are potential energy diagrams depicting the relative potential energies of electrons at different locations within the device of the present invention when the voltage applied to the electron extracting electrode 22 is 100 volts and the voltage on the phosphor coated screen 50 is 20,000 volts. Point 64 represents the potential energy of electrons at the tiplets 18, point 66 is the potential energy of electrons at the electron extracting electrode 22, point 68 is the potential energy of electrons at the screen electrode 52 and point 70 is the potential energy of electrons at the phosphor coated screen 50. FIG. shows the electron potential energy diagram for the device when the switch 58 is in position A. In this position, the voltage on the field emitting element 14 is +5 volts and the voltage on the screen electrode 52 is 5 volts. Since the potential energy of electrons at the screen electrode 52 is higher than the potential energy of electrons at the field emitting element 14, as represented by points 68 and 64 respectively in FIG. 15, electrons which have been extracted from the field emitting element will not be able to transit the screen electrode toward the phosphor coated screen 50 because the region of higher potential energy acts as a barrier to the passage of the electrons. When the switch 58 is placed in position B, the voltages on the field emitting element 14 and the screen electrode 52 are reversed resulting in an electron potential energy relationship as shown in FIG. 16. Since the potential energy of electrons at the field emitting element 14 is now higher than the potential energy of electrons at the screen electrode 52, as represented by points 64 and 68 respectively in FIG. 16, the potential energy barrier has been eliminated and electrons which have been extracted from the field emitting element will pass through the screen electrode and strike the phosphor coated screen 50 thereby producing light.

Referring to FIG. 17, represents the capacitance between the screen electrode 52 and the field emitting element 14. A signal having a positive voltage with respect to the field emitting element is momentarily applied to the screen electrode causing the capacitance C to charge up to a voltage corresponding to the value of the applied signaL yoltage. After the capacitance C has been charged, the potential energy of electrons at the screen electrode, as represented by point 68 (a) in FIG. 18, is less than that at the field emitting element 14 as indicated by point 64 and reference line 74. With an appropriate voltage, for example 100 volts, applied to the electron extracting electrode 22, some of the electrons emanating from the field emitting element 14 will transit the screen electrode 52 and strike the phosphor coated screen 50 while other will strike the screen electrode causing the voltage stored on the capacitance C to be reduced. As more electrons strike the screen electrode, the voltage on the capacitance C will be further reduced until the potential energy of electrons at the screen electrode, as represented by point 68(1)), is substantially equal to that at the field emitting element 14 at which time further passage of electrons through the screen electrode will cease. Consequently, the quantity of electrons striking the phosphor coated screen 50 can be regulated by varying the voltage of the applied signal.

One of the major advantages of the device of the present invention over prior art devices are the improved structural stength and heat transfer characteristics afforded by the combination of a slender tiplet on a relatively large pyramidal or conical base. Another advantage lies in the enhanced reliability afforded by the presence of a plurality of tiplets at each emission site as opposed to a single point which is characteristic of the prior art. The failure of a single tiplet will not appreciably degrade the performance of a particular site as would the failure of a pointed tip in a prior art device. Still another advantage accrues from the method disclosed herein which permits the fabrication of large (on the order of 10*) quantities of uniform emission sites over a large area (on the order of 10ft. Additional uniformity of emission from site to site is obtained by the use of a screen electrode as disclosed herein.

The device of the present invention, when embodied as an array of field emitting elements, can be used in those applications requiring a large area cathode having a plurality of electron sources such as the Electron Deflection System for Image Reproduction disclosed in U.S. Pat. No. 3,176,184, the Electron Beam Scanning Device disclosed in U.S. Pat. No. 3,539,719 or the Plural Beam Electron Beam Scanner Utilizing a Modulation Grid disclosed in U.S. Pat. No. 3,708,713 for example. In addition, the display device disclosed herein,

having an electron source comprising an array of field emitting elements, can be used as a programmable display device by selectively generating one or more streams of electrons and controlling the quantity of electrons which impinge upon the phosphor coated screen. The selective generation of electron streams may be accomplished, for example, utilizing strips of field emitting elements disposed in a matrix relationship with screen electrode strips. Generation of an electron stream can be effected from any desired field emitting element by causing the proper differential voltage to be applied between said field emitting element and the intersecting screen electrode strip. Modulation of the quantity of electrons striking the phosphor coated screen can be accomplished by, for example, selectively applying a signal voltage to the intersecting screen electrode strip in order to charge the associated screen electrode emitting element capacitance for the purposes of controlling the passage electrons as described above.

I claim:

I. A non-thermionic field emitting device comprising:

a. an electrically conductive substrate having 0pposed surfaces;

b. at least one field emitting element on said one surface of said substrate, said field emitting element having a pointed tip which projects away from said substrate; and

c. at least one needle-like projection disposed only on said pointed tip of said field emitting element whereby electron emission is enhanced by convergent lines of force of an electric field.

2. A non-thermionic field emitting device in accordance with claim 1 in which said field emitting element has a substantially pyramidal shape.

3. A non-thermionic field emitting device in accordance with claim 2 having an electron extracting electrode insulatingly spaced from and substantially parallel to said one surface of said substrate, said electron extracting electrode comprising a layer of electrically conductive material having at least one aperture therein which is positioned substantially coaxially with a corresponding field emitting element.

4. A non-thermionic field emitting device in accordance with claim 3 having a plurality of field emitting elements in a matrix array on said one surface of said substrate.

5. A device for modulating the flow of electrons comprising the following in a vacuum tight relationship:

a. an electron source comprising at least one field emitting element having a pointed tip with at least one needle-like projection disposed only on said pointed tip;

b. an electron target;

c. a modulating element positioned beetween said electron source and electron target to control the quantity of electrons impinging upon said electron target, said modulating element having a known capacitance with respect to said electron source;

d. means whereby said known capacitance can be charged to a predetermined voltage level;

e. an electrode for extracting electrons from said field emitting element, said electrode in insulatingly spaced-relation between said field emitting element and said modulating element and structured such that the electron extracting electrode will not impede the passage of the electrons from said field emitting element to said modulating element.

6. A device in accordance with claim 5 in which said electron extracting electrode comprises a layer of electrically conductive material having at least one aperture therein in coaxially spaced relation with a corresponding field emitting element.

7. A device in accordance with claim 6 in which the modulating element is a screen electrode parallel to and insulatingly spaced from said electron extracting electrode, said screen electrode comprising a sheet of an electrically conductive material having a mesh-like structure formed by a multiplicity of finely spaced apertures.

8. A device in accordance with claim 7 in which the electron target comprises a phosphor coated display screen.

9. A display device comprising the following in a vacuum tight relationship:

a. a plurality of field emitting electron sources, each field emitting electron source having a pointed tip and at least one needle-like projection disposed only on said pointed tip;

b. an electron target;

c. a modulating element disposed between each of said electron sources and said electron target to control uniformity of the quantity of electrons impinging upon said electron target from each source, each modulating element having a known capacitance with respect to its associated electron source; and

d. means whereby each known capacitance can be charged to a predetermined voltage level.

10. A display device in accordance with claim 9 in which said plurality of electron sources comprises a matrix array of electron emitting sites, each site comprising:

a. a field emitting element disposed on an electrically conductive substrate, said field emitting element having a pointed tip which projects away from said substrate and at least one needle-like projection disposed only on said tip; and

b. an electrode for extracting electrons from said field emitting element, said electrode comprising a layer of an electrically conductive material disposed in insulatingly spaced relation between said field emitting element and said modulating element, said layer having an aperture therein in coaxially spaced relation with said field emitting element.

11. A display device in accordance with claim 10 in which said modulating element is a screen electrode disposed parallel to and insulatingly spaced from said electron extracting electrode, said screen electrode comprising a layer of an electrically conductive material having a mesh-like structure formed by a multiplcity of finely spaced apertures.

12. A display device in accordance with claim 11 in which the electron target comprises a phosphor coated display screen.

UNITED STATES PATENT OFFI(.E CETIFICATE OF CORRECTiON PATENTNO. 3,921,022

DATED November 1 1975 |NVENTOR(S) Jules David Le It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 35, after "FIG. 17," insert C.

Signed and Sealed this thirtieth D f March 1976 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ummissimur o l'larenrs and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO 3 921 022 DATED 3 November 18, 1975 INVENTOWS) 1 Jules David Levine It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 35, after "FIG. 17," insert C.

Signed and Sealed this thirtieth D f March 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Patents and Trademarks 

1. A non-thermionic field emitting device comprising: a. an electrically conductive substrate having opposed surfaces; b. at least one field emitting element on said one surface of said substrate, said field emitting element having a pointed tip which projects away from said substrate; and c. at least one needle-like projection disposed only on said pointed tip of said field emitting element whereby electron emission is enhanced by convergent lines of force of an electric field.
 2. A non-thermionic field emitting device in accordance with claim 1 in which said field emitting element has a substantially pyramidal shape.
 3. A non-thermionic field emitting device in accordance with claim 2 having an electron extracting electrode insulatingly spaced from and substantially parallel to said one surface of said substrate, said electron extracting electrode comprising a layer of electrically conductive material having at least one aperture therein which is positioned substantially coaxially with a corresponding field emitting element.
 4. A non-thermionic field emitting device in accordance with claim 3 having a plurality of field emitting elements in a matrix array on said one surface of said substrate.
 5. A device for modulating the flow of electrons comprising the following in a vacuum tight relationship: a. an electron source comprising at least one field emitting element having a pointed tip with at least one needle-like projection disposed only on said pointed tip; b. an electron target; c. a modulating element positioned beetween said electron source and electron target to control the quantity of electrons impinging upon said electron target, said modulating element having a known capacitance with respect to said electron source; d. means whereby said known capacitance can be charged to a predetermined voltage level; e. an electrode for extracting electrons from said field emitting element, said electrode in insulatingly spaced-relation between said field emitting element and said modulating element and structured such that the electron extracting electrode will not impede the passage of the electrons from said field emitting element to said modulating element.
 6. A device in accordance with claim 5 in which said electron extracting electrode comprises a layer of electrically conductive material having at least one aperture therein in coaxially spaced relation with a corresponding field emitting element.
 7. A device in accordance with claim 6 in which the modulating element is a screen electrode parallel to and insulatingly spaced from said electron extracting electrode, said screen electrode comprising a sheet of an electrically conductive material having a mesh-like structure formed by a multiplicity of finely spaced apertures.
 8. A device in accordance with claim 7 in which the electron target comprises a phosphor coated display screen.
 9. A display device comprising the following in a vacuum tight relationship: a. a plurality of field emItting electron sources, each field emitting electron source having a pointed tip and at least one needle-like projection disposed only on said pointed tip; b. an electron target; c. a modulating element disposed between each of said electron sources and said electron target to control uniformity of the quantity of electrons impinging upon said electron target from each source, each modulating element having a known capacitance with respect to its associated electron source; and d. means whereby each known capacitance can be charged to a predetermined voltage level.
 10. A display device in accordance with claim 9 in which said plurality of electron sources comprises a matrix array of electron emitting sites, each site comprising: a. a field emitting element disposed on an electrically conductive substrate, said field emitting element having a pointed tip which projects away from said substrate and at least one needle-like projection disposed only on said tip; and b. an electrode for extracting electrons from said field emitting element, said electrode comprising a layer of an electrically conductive material disposed in insulatingly spaced relation between said field emitting element and said modulating element, said layer having an aperture therein in coaxially spaced relation with said field emitting element.
 11. A display device in accordance with claim 10 in which said modulating element is a screen electrode disposed parallel to and insulatingly spaced from said electron extracting electrode, said screen electrode comprising a layer of an electrically conductive material having a mesh-like structure formed by a multiplcity of finely spaced apertures.
 12. A display device in accordance with claim 11 in which the electron target comprises a phosphor coated display screen. 